Crude oil reserve is rapidly depleting. At present, cultivation of photoautotrophic organisms such as microalgae has been identified as a potential strategy to address the aforementioned major concerns. Microalgae may be used as an alternative feedstock for biofuel production. Unlike the rapidly depleting cruel oil reserve, it is renewable and provides rapid growth. With high lipid triglyceride content up to 80% of the dry weight and rapid exponential growth rates, microalgae potentially offer high oil productivities at low raw material costs, in contrast to high vegetable oil costs commonly observed in biodiesel production. In addition to biodiesel production, microalgae are presently cultured to produce materials for health supplements, aquaculture feed, pigment components, polyunsaturated fatty acids and other fine chemicals, while the waste cellular debris, collected after harvesting of valuable lipids and materials, can be converted into alternative biofuel forms including biomethane, bioethanol and biohydrogen.
Cultivation of microalgae can be classified under two major systems: open system (e.g. raceway ponds) or closed systems (e.g. photobioreactors). Presently, open systems remain the more popular cultivation system industrially due to the low cost associated with building and operation of the open systems. However, the key disadvantage of open raceway ponds is that biomass areal and volumetric productivity is low. In addition, raceway ponds are susceptible to contamination and are poorly mixed. Therefore, they are unable to utilize light efficiently and use carbon less effectively than closed systems.
In contrast, photobioreactors enable single-species culture to be produced with a higher volumetric productivity, and photobioreactors have been used for industrial production of large quantities of microalgal biomass. Furthermore, in order for high-value compounds to be produced, it mandates the use of monocultures in controlled cultivation systems. A conventional photobioreactor is capable of sustaining higher biomass concentration than that obtained in open raceway ponds. This hence minimizes the size requirement of the reactor, and simultaneously lowers the downstream processing costs.
Tubular photobioreactors with small diameters are currently employed in large-scale production of microalgae. While small diameter tubes are found to maximize the utilization of solar light capture, strong turbulent streaming is required, and additionally, the high pumping pressure required for maintaining the necessary flow rate through the length of the tube render the system more expensive and less practicable for large scale operations. Furthermore, small tube diameters have also been found to increase bio-fouling, reduce control over salt precipitation, and possess strong oxygen tension that is disadvantageous in general for actively photosynthesizing systems.